1. Field of the Invention
The invention relates to a process for absorbing chlorine from gas streams in water with regulated addition of hydrogen peroxide.
2. Description of the Related Art
Industrial processes in which chlorine compounds are exposed to high temperatures (for example combustion of organochlorine compounds, chlorination reactions and hydrolysis reactions) form hydrogen chloride, which forms elemental chlorine with any oxygen present according to the Deacon equilibrium. This elemental chlorine is present in the offgas stream and has to be removed therefrom to comply with emissions limits.
Since the solubility of chlorine in water is low, chlorine is generally absorbed with basic absorbents, such as sodium hydroxide solution as described below by equations (1) and (2):Cl2+H2O - - - >HCl+HClO  (1)HCl+HClO+2NaOH - - - >NaCl+NaClO+H2O  (2)However, other basic absorbents (milk of lime, ammonia, amines, carbonates, hydrogencarbonates, etc.) are also possible.
In all basic absorption processes, the driving force is the formation of salts (chloride and hypochlorite). The result of these processes is formation of hypo-chlorite-containing salt solutions which find a limited range of applications (e.g. chlorine bleach) or are added to the wastewater after appropriate treatment. For this purpose, the hypochlorites formed must be reduced, which is typically done by adding sulfites, thiosulfates, hydrogen peroxide inter alia. For instance, in EP0406675, an absorption solution consisting of sodium hydrogencarbonate and sodium hydrogensulfite is used.
Absorption in acidic media has also been described. In this case, usually metal salts in low oxidation states (e.g. Fe2+, Cu1+, etc.) are used as absorbents, and are then converted to higher oxidation states by chlorine (see, for example, DE-A 2545342).
All processes described above form salt solutions which only have limited further utility.
Additionally described is the separating-out of chlorine with organic solvents. For instance, compounds such as tetrachloromethane, dichlorotoluene, dichlorobenzotrifluoride, etc. are used in this process. These substances have the disadvantage of possessing a noticeable vapor pressure, and have to be removed in turn from the cleaned gas streams.
SU 697160 describes a method for cleaning offgases which comprise boron chloride, titanium chloride, and also chlorine, hydrogen chloride and phosgene. This method requires concentrated hydrochloric acid in one stage and hydrogen peroxide in hydrochloric acid solution in a further stage as absorbents. This method is further characterized in that it works without waste and hence without wastewater. What is described is a batch process in which a concentrated 35% hydrochloric acid and a 9-30% hydrogen peroxide solution are first initially charged and then the offgas is passed through.